This invention relates to method and apparatus for creating multiple level phase structures in a substrate.
Certain diffractive optical elements employ spatially varying structures which alter the phase of transmitted light. An example is the Fresnel zone plate which consists of concentric annular regions, each region adapted to alter, by a desired amount, the phase of light passing through the region. The variation in phase results from a change in the index of refraction or the thickness of the material in each of the annular regions.
One known technique for altering the phase is by varying the thickness of a transparent substrate, such as glass, in correspondence with a desired phase profile. Such surface relief or kinoform structures can be fabricated by etching or by the deposition of material. Generally, a surface relief diffractive optical element having ideal characteristics would include relief structures defined by smooth profiles which are not readily fabricated with current technology. Therefore, the ideal smooth profiles are often approximated by a staircase arrangement of multiple discrete levels.
Surface relief encoding is often achieved by serial masking and etching operations performed on a glass substrate. See, for example, J. Jahns et al., "Multilevel phase structures for array generation," Proc. SPIE 1052, pp. 198-203 (1989) and G. L. Swanson, "Binary optics technology: the theory in design of multi-level diffractive optical elements" MIT Lincoln Lab. Tech. Report 854, (1989). The techniques described in these articles primarily utilize a mask and etch process in which "N" masks are required to create 2.sup.N discrete phase levels. The masks, created with electron beam lithography, are capable of producing small feature sizes down to 0.2 .mu.m. The theoretical efficiencies of Fresnel lenses produced with this technique are very high, namely, 81 percent for four levels, 95 percent for eight levels and 99 percent for sixteen levels (not including insertion losses). Computer generated holographic techniques can be used to design arbitrary phase profiles, and large numbers of these discrete phase level structures can be made by embossing plastic-based substrates with dies created from the etched glass substrate master. See, P. A. Mokry, "Unique applications of computer generated diffractive optical elements" Proc. SPIE 1052, pp. 163-171 (1989). This masking and etching process is very capital intensive, requiring a clean-room environment, specialized machinery and caustic chemicals. The turnaround time is also quite long.
Another approach for creating computer generated holograms (CGHs) is binary phase encoding with laser printer-aided fabrication. This approach, however, yields low efficiency, off-axis holograms. One known form of CGH is the binary amplitude CGH which creates specified amplitude and phase variations by selective blocking and passing of light. See, for example, G. Tricoles, "Computer generated holography: a historical review", Applied Optics, 26, pp. 4351-4360 (1987). This technique is readily implemented with a personal computer, a conventional 300 dot per inch laser printer, and off-the-shelf high-resolution camera equipment. However, since this technique varies amplitude in a binary fashion, most of the incident light is blocked or wasted in unwanted diffraction orders. Thus, this technique suffers from low diffraction efficiency (a maximum of 10.1 percent in the Fresnel zone lens case) leaving it highly impractical for use in cascaded optical systems. See, C. W. Han, "Experimental analysis of error effects on computer generated hologram performance", Masters Thesis, Carnegie Mellon University (1987).
Another technique for creating CGHs is to use a discrete gray level plotter to implement fringes that are similar in nature to those created optically and recorded in holographic film. See, P. Anastasi et al., "Evaluation of coding schemes for dedicated beam shaping CGH," I.E.R.E. Conference Holographic Systems, Components and Applications (Cambridge, UK 1987), pp. 81-85 and L. Lindvold, "Linearisation problems in computer-generated holographic optical elements with grey-level modulation," Journal of Physics D, 22, pp. 735-740 (1989). Little attention has been paid to this technique which requires specialized gray level plotters and results in low diffraction efficiency because of its amplitude nature.